Vehicle fuel systems include evaporative emission control systems designed to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the vapors to be purged into the engine intake manifold for use as fuel.
In hybrid-electric vehicles, the fuel tank is typically sealed with a fuel tank isolation valve (FTIV). Prior to refueling, fuel vapors in the tank may be vented to the fuel vapor canister by opening the FTIV. When the fuel tank pressure reaches a safe level, access to the fuel filler neck may be granted, through unlocking a refueling door, for example.
However, opening the FTIV when a significant pressure exists within the fuel tank may result in the corking of other fuel tank valves such as a grade vent valve and/or a fill limit vent valve, rendering them unusable. Depressurizing the fuel tank in this fashion could also lead to an abundance of hydrocarbons entering the engine intake system, altering the air/fuel ratio, and potentially leading to engine stalling. Additionally, any delays or imprecision in control of an FTIV may increase release of hydrocarbons or degraded air-fuel ratio control during purging. Further a loud hissing noise may occur as the fuel tank is depressurized in a single stage. Dual stage FTIVs with multiple sized orifices, or the use of multiple valves in parallel have been described, but these configurations may add complexity to a system, and may increase the difficulty of diagnosing malfunctions within the system.
The inventors herein have recognized the above problems and have developed systems and methods to at least partially address them. In one example, a method for a fuel system, comprising: during a first condition, fluidly coupling a fuel tank to a fuel vapor canister; and fluidly coupling the fuel vapor canister to atmosphere via a deactivated vacuum pump. By fluidly coupling the fuel vapor canister to atmosphere via a deactivated vacuum pump, the fuel tank may be depressurized slowly without requiring a multi-stage fuel tank isolation valve. In this way, the cost and complexity of the fuel system is reduced.
In another example, a system for a hybrid-electric vehicle, comprising: a fuel tank isolation valve coupled in a conduit between a fuel tank and a fuel vapor canister; an evaporative leak check module coupled in a conduit between the fuel vapor canister and atmosphere, the evaporative leak check module comprising a vacuum pump and a changeover valve operable between a first and second conformation, the evaporative leak check module; and a controller configured with instructions stored in non-transitory memory that when executed cause the controller to: responsive to a refueling request and further responsive to a fuel tank pressure being greater than a first threshold: place the changeover valve in the second conformation without activating the vacuum pump; and open the fuel tank isolation valve. The system thus controls the rate of depressurization via the evaporative leak check module changeover valve. In this way, the fuel tank isolation valve may be opened completely during depressurization without risking the fuel limit vent valve aspirating shut.
In yet another example, a method for refueling a vehicle, comprising: responsive to a refueling request, and during a condition where a fuel tank pressure is greater than a first threshold, opening a fuel tank isolation valve coupled between a fuel tank and a fuel vapor canister; fluidly coupling the fuel vapor canister to atmosphere via a deactivated vacuum pump; and responsive to the fuel tank pressure decreasing below the first threshold, fluidly coupling the fuel vapor canister to atmosphere bypassing the deactivated vacuum pump. By changing the position of the vent pathway between the fuel vapor canister and atmosphere, the fuel tank may be depressurized in two-stages without requiring additional components, such as a multi-valve fuel tank depressurization system. In this way, fuel system diagnostics may be simplified, leading to more robust diagnostic tests.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.